1. Field of the Invention
This invention generally relates to an aqueous solution of an alkaline degreasing agent, (hereinafter referred to as "an alkaline degreasing solution"), employed for removing oil and soil from surfaces of a steel sheet, galvanized steel sheet or galvannealed steel sheet, and an alkaline degreasing agent employed for making up and replenishing the alkaline degreasing solution. It relates more particularly to an alkaline degreasing solution with a low foaming tendency which are effective in a metal surface cleaning especially through a spray degreasing or immersion degreasing at a relatively low temperature such as 40.degree. C., provided prior to a chemical conversion treatment and paint coating therefor.
2. Description of the Prior Art
Conventionally, metal products made of the aforementioned metals are generally treated by the following steps prior to paint coating:
______________________________________ surface cleaning (degreasing) .dwnarw. water rinsing (multistage method) .dwnarw. surface conditioning .dwnarw. chemical conversion treatment .dwnarw. water rinsing (multistage method) .dwnarw. deionized water rinsing .dwnarw. drying off .dwnarw. paint coating; anionic electrocoating cationic electrocoating electrostatic coating powder coating spray or brush coating etc. ______________________________________
The alkaline degreasing solutions and alkaline degreasing agents employed for cleaning metal surfaces have been introduced in various literature.
The alkaline degreasing agents are generally a powdery mixture of alkaline builders (alkaline compounds which are a main component of an alkaline degreasing agent), surface active agents, and defoaming agents.
The alkaline degreasing solution in which the alkaline degreasing agent is dissolved is employed for removing oil and soil from metal surfaces by means of a spray cleaning or immersion cleaning, the temperature of the solution employed for the cleaning generally being 40.degree.-70.degree. C. and degreasing time 1-10 minutes.
Recently, an interest has been increasingly generated in the alkaline degreasing agents and alkaline degreasing solutions utilized at a low temperature, with a view to energy conservation. The alkaline degreasing agents made for this purpose, however, have had the following deficiencies when they have been used for cleaning surfaces of metals (to be treated):
a. excessive foaming during degreasing.
b. low foaming but poor degreasing effects.
In the case that an alkaline degreasing solution excessively foams;
1. The foam may spill out of a degreasing tank and cause a loss of the alkaline degreasing solution in a large quantity, and
2. The foam may enter the inside of a box section of the metal products or of a box shaped metal product at the outset of an immersion degreasing. The foam left inside, due to the above shapes of the metal products, hinders the alkaline degreasing solution from contacting with inner surfaces thereof, and the foam whereby causes an incomplete degreasing on the inner surfaces.